Such heat exchangers most usually find application in motor vehicles having internal combustion engines, and are used either as engine coolant radiators or as radiators in the heating system for the cabin of the vehicle. A coolant fluid, most commonly a mixture of water and glycol, flows through the tubes of the heat exchanger in a closed circuit, and is cooled by heat transfer with air passing through the heat exchanger and around the tubes.
The tubes conventionally have a circular transverse cross section, both in the body of each tube and in its end portions. However, with a view to optimising the heat transfer performance of this type of heat exchanger, and in particular with a view to reducing mechanical losses in the cooling air passing through the heat exchanger, it is also common practice to use tubes in which the body of the tube is of a non-circular transverse cross section, this being usually oblong, i.e. of a shape which generally defines a major axis and a minor axis. Such oblong tubes may be of the "flat", oval, or elliptical type. In such non-circular tubes, the end portion usually has a transverse cross section having a different shape from the cross section of the tube body. It will typically be less oblong than the body and may indeed be circular, to avoid any possible deformation of the end portion. In other words, in the end portion, the difference in length between the major and minor axes is less than that in the body of the tube, and may even be zero.
The end portions are open into a water box or manifold, with the collecting plate forming an inner wall of the water box. In order to separate the interior of the water box from the space in which air flows around the tubes, it is necessary to seal the end portions of the tubes in the collecting plate. Such sealing is often obtained in practice by means of an appropriate sealing gasket which is compressed between the end of the tube and a lip formed on the collecting plate and surrounding the corresponding hole in the latter through which the end portion of the tub extends. With this arrangement, the pressure exerted by the sealing gasket gives rise to a danger that the end portion of the tube may be squeezed towards a flattened shape.
Regardless of the cross sectional tube shape and type of collecting plate adopted, it is necessary that the amount of material in the collecting plate between two adjacent holes in the latter should be as small as possible consistent with the required mechanical strength of the plate. In other words, the pitch distance between adjacent holes in the plate should be minimised. The extent to which this can be done is of course limited by the need to form and accommodate the lips with which the holes in the connecting plate are normally bounded.
Under these conditions it is difficult to optimise the pitch distances between the various tubes, having regard to the different criteria which must be satisfied to produce a satisfactory heat exchanger design, and in particular to produce a satisfactory collecting plate.